Catheters connected to inject a medicinal fluid into a patient's cardiovascular system are subject to occlusion as blood clots or thrombi form within the catheter passage. To minimize the occurrence of this problem, a very low flow rate of the fluid is continuously maintained through the catheter from a source container. Periodically, medical personnel must momentarily ope a valve to create a substantially greater flow rate of fluid to flush incipient thrombi from the catheter. In addition, this flushing flow of fluid may be momentarily enabled prior to connection of the catheter to the patient, in order to evacuate air from the line and fill it with the fluid. In the medical field, a device used to restrict fluid flow and selectively provide a flushing flow through a catheter is known as "flush valve." A typical flush valve contains a very small diameter capillary flow passage that limits fluid flow to the desired low rate. Typically, the relatively greater flushing flow is effected in such devices by opening a separate parallel path that does not include the restriction of the capillary flow passage.
Exemplary of prior art flush valves is the design disclosed in U.S. Pat. No. 4,192,303. In this flush valve, a flexible conduit defines a passage between an inlet and an outlet. Positioned coaxially in the flexible conduit is a cylindrical plug member that has a raised band intermediate its ends. The diameter of the raised band is sufficient to seal against the inner surface of the flexible conduit, normally limiting fluid flow through a marine bore formed through the longitudinal center of the plug. To produce the larger flushing flow, a user squeezes the flexible conduit, causing a passage to open between the raised band and the inner surface of the conduit. Fluid then flows through the larger passage, bypassing the flow retriction normally imposed by the marine bore. When released, the flexible conduit reseals against the raised band, closing the bypass passage and shutting off flow of fluid through it. Any obstruction in the marine bore capillary flow passage is not cleared by the flushing flow in such prior art flush valves, because the flushing flow is completely separate from the capillary flow passage. Consequently, obstruction of the marine bore may eventually allow thrombi to block the catheter downstream of the flush valve.